Power transmission belts having a variety of groove and rib configurations are known. One such belt is a multi-ribbed belt. The multi-ribbed belts have a tension section, a load carrying section, and a compression section. Multi-ribbed belts may also be provided with transverse grooves; such transverse grooves extending either traverse to the belt or at an angle relative to the traverse direction of the belt. The longitudinal and transverse grooves are located in the compression section. Such belts are known as cogged multi-ribbed belts, examples of which are disclosed in U.S. Pat. Nos. 4,002,082 and 5,382,198. Cogged multi-ribbed belts exhibit improved flexibility and longer life.
However, cogged multi-ribbed belts create more noise due to a non-continuous rib entering and exiting the grooved pulley. Noise is generated when the tooth travels and presses into the pulley groove, compressing and displacing the air in the groove and noise is generated when the tooth exits the pulley groove as air rushes to fill the now empty pulley groove. Furthermore, there are harmonic noise spikes generated by the cogs at the cog engagement frequency.
Two methods are known to reduce the noise of a cogged multi-ribbed belt. The first is to incline the transverse grooves at an angle relative to the transverse direction. This reduces the overall noise level but the harmonic noise spikes are often still objectionable.
The second is to vary the pitch of the cogs with a repeating pitch pattern, as disclosed by U.S. Pat. Nos. 4,262,314 and 4,832,670. U.S. Pat. No. 4,262,314 discloses a cog belt with reduced noise. The transverse groove depths, the groove angles, and the distance between the grooves are varied. Similar to U.S. Pat. No. 4,262,314, U.S. Pat. No. 4,832,670 also discloses multiple elements of the belt construction are varied simultaneously to produce a reduced noise belt. The belt is defined by a repeating sequence pattern along the length of the belt. An ideal pitch pattern that results in a minimization of noise, however, needs to vary based on the circumferential length of the belt. Thus, to optimize reduction of noise in belts of varying length, each belt would require a unique respective pitch sequence or pattern.
The manufacture of a cogged, multi-ribbed belt may be effected in a plurality of processes common to the industry. U.S. Pat. Nos. 4,575,445 and 4,512,834 illustrate and describe two such manufacturing processes and are representative of the manufacture of a cogged belt from one or more molds. A third alternative process begins with a metal preform board. The cog profile is machined into the board and a rubber matrix is made from the preform board. This matrix is then spliced on the outside of green diaphragm rubber to form a diaphragm mold. The diaphragm molds hence have an external cog profile identical to the actual belt and the rubber diaphragm molds are then used to make diaphragms. Belt materials are plied on a build mandrel and dropped into a diaphragm in a cure pot. After a slab of the belts have been formed and cured, “v's” are milled and slit into individual belts. The three processes referenced above are not exclusive and other cogged belt manufacturing methods are known and utilized in the industry. Common to numerous known approaches is that one or more molds are used for the purpose of creating the cog sequence in a belt. For belts having a repeating or random cog sequence, the molds used in the manufacture thereof must mirror the desired cog profile and pattern.
From the foregoing, it will be appreciated that the creation of one or more molds for the purpose of manufacturing a cogged belt, by any of the processes known and practiced in the industry is an investment of capital and, hence, expensive. Since a mold is cog-profile specific, that is, identical to the cog profile desired in the actual belt, belts having cogs of differing profiles or pitch sequencing are typically formed from a mold unique to the belt.
As mentioned above, it is often desirable to utilize a repeating pitch pattern to reduce the noise generated from a cogged multi-ribbed belt. However, the ideal pitch pattern on a belt needs to vary based on the circumferential length of the belt. Heretofore, in order to manufacture a belt of a given circumferential length, a unique, dedicated mold providing the requisite optimized pitch sequence was required. Because the creation of a unique mold for each length of belt is cost prohibitive, the industry practice has been to been slow to adopt noise reduction techniques in cog design and sequence in belts of varying length. While this practice avoids the costly proliferation of unique, belt-length dependent, manufacturing molds, optimized noise reduction is sacrificed.